The field of high-frequency ultrasound (“HFU”) imaging, using frequencies above 20 MHz, is growing rapidly as transducer technologies improve and the cost of high bandwidth electronic instrumentation decreases. Single element focused transducers, however, are currently used for most HFU applications. These single element transducers are limited in their application due to their inherent small depth of field, which limits the best image resolution to a small axial range close to the geometric focus of the transducer.
HFU transducers primarily utilize single element focused transducers fabricated with polyvinylidene fluoride (“PVDF”) membranes as their active acoustic layer. These transducers are relatively simple to fabricate but suffer from a fairly high two-way insertion loss (≈40 dB) because of the material properties of PVDF. As a result, methods have focused on improving the insertion loss by optimizing the drive electronics and electrical matching. Single element PVDF transducers continue to be the primary transducer choice for HFU applications and have been fabricated using a ball-bearing compression method.
Similarly, methods of fabricating single element HFU transducers using ceramic material have been refined. A number of ceramic devices have been fabricated successfully to operate in the HFU regime. Ceramic devices have an inherent advantage over PVDF based transducers because of their low insertion loss. Ceramic materials, however, are typically used for flat arrays because they are difficult to grow or to press into curved shapes. Fabricating HFU ceramic transducers into concave shapes is known in the art through the use machining, coating, lapping, laminating and/or heat forming techniques for bonding and shaping curved transducers. These known fabrication techniques are used to construct single element transducers, and are not used to construct an array transducer.
Both PVDF and ceramic transducers have been used to great success for ophthalmic, dermatological, and small animal imaging. Current methods aim to fabricate individual array elements on the order of λ/2; these small dimensions necessitate advances in interconnects and electronics to fully implement the technologies. Accordingly, there exists a need for a technique for the feasible design and fabrication of a high frequency annular array transducer.